Conventional imaging systems employ intensity-based techniques to handle electromagnetic energy proceeding from a source (e.g., an object). As one example of a conventional system, a spectroscopic system determines spectral (wavelength) composition of objects and scenes. The wavelengths that comprise the collected energy are separated with the use of a dispersive element employing refractive means, such as a prism, or diffractive means, such as a grating. After passing through one of these dispersive elements, the different wavelength components of the wave front propagate in different directions and the intensities of the components are recorded by an array of detector pixels. Such a standard spectrometer is an excellent device for determining the spectral composition of light emanating from a source object, but is unable to maintain two-dimensional spatial integrity of the source object. Typically, such a spectrometer is not capable of determining spectral content on a pixel-by-pixel basis, and merely collects the total intensity of electromagnetic energy proceeding from an object.
Fourier transform and Fabry-Perot interferometer systems are capable of performing imaging spectrometry and determining the spectral composition of an object on a pixel-by-pixel basis. However, there are certain limitations imposed by the geometry of these systems. For example, in both types of systems, field of view of is severely restricted.
For the Fourier transform interferometer, the length of the system, combined with the small size of the mirrors, restricts the field of view because optical rays will not propagate through the system for large angles. Therefore, the number of pixels that can be acquired is limited.
For the Fabry-Perot interferometer, a small field of view is the result of two main effects. First, the light coming from the source object undergoes multiple reflections within a mirrored optical cavity before emerging from the system. When the incident light comes from an off-axis point on the object, it enters the cavity at an incident angle other than zero. Consequently, as the light undergoes multiple reflections, it will “walk” along the mirrors and eventually leak out of the cavity. The result of this behavior is that, as the field of view increases, the energy throughput of the system decreases.
The second problem that results in a limitation of the field of view for the Fabry-Perot system has to do with band pass variation with field size. Since the effective mirror separation changes with field angle, so does the filter band pass. To minimize the spectral variation from the center to the edge of the field, the field of view has to be small. However, this will again limit the number of pixels that can be obtained.
Still another problem that can arise with respect to some known systems, such as the Fourier transform interferometer, deals with image registration. Typically, two-dimensional images are acquired as one mirror is scanned. Problems associated with scanning, such as mirror jitter, uneven scanning, or mirror walking, create registration problems between the images in the different spectral bands.
In addition, many known systems employ scanning to acquire the spectral composition of the electromagnetic energy proceeding from a source object. During such scanning, it difficult to obtain the spectral composition in real-time while maintaining a high signal-to-noise ratio. This is not only a problem for the Fourier transform and Fabry-Perot interferometers, but also for electrically scanned systems such as liquid crystal systems and acousto-optic tunable filter based imaging spectrometers, which have the additional problem of relatively low transmission.
Tomographic-based methods are sometimes used for imaging spectrometry tasks. Tomographic methods negate the need for scanning. However, the downside of this technique is that it is computationally intensive, requiring the mathematically determination of a system matrix, which is usually application specific.
As mentioned above, conventional imaging techniques employ intensity collection techniques. However, it is to be noted that, in distinction, spatial phase is intensity independent. Spatial phase characteristics of electromagnetic energy include characteristics of the plurality of polarizations (e.g., linear and circular) that are present within the electromagnetic energy.
As one type of utilization of polarization characteristics, polarimetry identifies, isolates, and/or uses a generalized polarization of electromagnetic energy. In the past, scientists have used polarimetry to filter imagery for specific applications. Polarization filters are used to collect polarization data, and classical polarization theory is used to determine one level of the spatial phase properties. However, overall spatial phase of a propagated electromagnetic wave can a significant amount of information that is indicative of unique features about the wave history. For example, as properties of an electromagnetic wave change as the wave interacts with media and changes as the wave transverses a surface.
In the past, scientists have attempted to build operational imaging polarimeters. None have been successful in providing an operation device that has abilities greater than a two channel orthogonal system. The polarimeters to date have been limited to a single set of four detectors or a rotating polarization analyzer. The rotating analyzer limits the system to static scenes and is not a useful tool for spatial phase analysis.
Another problem that arises for imaging systems that employ moving components, such as a rotating member, deals with the issue of image registration. However, problems associated with rotating, such as optical wedge wobbling, uneven rotating, or beam walking, create registration problems between the images in the different phase channels. With spatial phase imaging, it is critical that each channel is identical in spatial content as well as angular information. Rotating systems will vary the angular extent of the object and cannot be used effectively. Therefore, while some of the prior art is capable of performing limited polarimetry and other intensity-based applications, it is not capable, for the reasons discussed, of providing true, multi-dimensional, real-time spatial phase imaging.
The inventor has recognized that a spatial phase system would solve the above-mentioned problems and also gone further into the complete analysis of the phase information, which is contained in the electromagnetic energy. By the scientific analysis of all the radiation being transmitted, reflected, emitted and/or absorbed, one can determine its phase properties. The phase properties are those characteristics that convey information (e.g., an indication of the media through which a wave has passed) that could allow significant imaging abilities. Along these lines, the inventor has recognized that spatial phase is a technology with tremendous benefit potential.